ES6 destructuring with this variable - tuple and array

[Guardiannw]

TypeScript Version: 1.8.10
Code

// A *self-contained* demonstration of the problem follows...
var array = [1, 2, 3];
[this.test, this.test2, this.test3] = array;

Expected behavior:
for it to compile as _this.test = array[0], _this.test2 = array[1], _this.test3 = array[2];
Actual behavior:
it compiles as _this.test = array[0], this.test2 = array[1], this.test3 = array[2];

This is a problem because the this scope does not match correctly that of the lambda.

[RyanCavanaugh]

Can't repro on the Playground this with the code given. Can you post a complete sample (with the surrounding function) ?

[Guardiannw]

@RyanCavanaugh I could not reproduce this on the playground either. But strangely i figured out a fix.

When I would write something to the effect of

wrapper((array: [any]) => {
    [this.test, this.test1, this.test2] = array;
});

it would fail and would output as I had previously stated.
But when I changed the type, it worked correctly ->

wrapper((array: Array<any>) => {
    [this.test, this.test1, this.test2] = array;
});

I am not sure why this is. I thought the type did not change transpilation, but only is there for transpile time type checking, but it looks like it matters for that too.

[RyanCavanaugh]

Definitely a bug

declare function wrapper(x: any);
wrapper((array: [any]) => {
    [this.test, this.test1, this.test2] = array;
});

should output three _thiss

[yuit]

@Guardiannw the reason why
this example

wrapper((array: [any]) => {
    [this.test, this.test1, this.test2] = array;
});

and this one

wrapper((array: Array<any>) => {
    [this.test, this.test1, this.test2] = array;
});

behaves differently is that the first example array has type tuple with one element (that is type any); when compiler type-check destructing assignment for the second element in tuple, it couldn't get type for that element so this expression isn't properly check (hence the flag to determine whether to capture lexically this doesn't get check as well). The second example is just an array so it doesn't affect by the similar issue

[Guardiannw]

@yuit That makes complete sense, but I thought that the types did not create any changes in the exported code, that they only made compile time type checks. Is this correct or incorrect with the desired specs?

[yuit]

@Guardiannw I think this is incorrect and undesirable that we skip checking this expression and doesn't set the flag properly. Emitted code of both cases should be the same although one will give you a compiler error.

I have merged the fix into release-2.0 and master if you would like to give it a try!